This invention relates to a controller for a two-stage heat source that can be used with either a single or a two-stage thermostat.
There are two types of commonly available, gas-fired, warm air furnaces in the marketplace: those with a single gas flow rate, and those with two or more gas flow rates. These are referred to as single and multistage furnaces, respectively. Multistage furnaces are frequently selected by homeowners for replacement furnaces because they offer increased performance and comfort. In retrofit applications there is typically a thermostat and wiring in place. It can be troublesome to install a multistage thermostat in a retrofit application with a single stage thermostat already in place because of the need to add additional wiring. For simple and economical installation, it is desirable to be able to continue to use a single stage thermostat and single stage thermostat wiring with a replacement multistage furnace.
Several attempts have been made to utilize a single stage thermostat with two-stage furnaces. One method, disclosed in U.S. Pat. No. 5,271,556, ties the first stage and second stage terminals on the controller together. The controller recognizes a simultaneous call for first stage and second stage heat as a single stage thermostat, and operates in the first stage only. If the demand for heat is not satisfied within a predetermined time (typically ten minutes), the controller switches to second stage heating. Another method, disclosed in U.S. Pat. No. 5,806,760, includes a time delay relay in the first stage terminal. After the expiration of the time for the time-delay relay, the controller switches to the second stage heating.